The present invention relates to the field of petroleum extraction, particularly drilling, both offshore and onshore, and essentially to a short flexible pipe, known as a “mud hose” or commonly as a “rotary hose” or “kelly hose”, which is used for injecting mud during rotary drilling operations. This is a flexible pipe that forms the link between the mud-injection head and the fixed pipe from the mud pumps, as described in the American Petroleum Institute's standards API 7L and API 7K. A hose of this type is illustrated, for example, in U.S. Pat. No. 4,514,103, to which reference may be made.
Mud hoses have to withstand high pressures, for example, a nominal in-service pressure of 350 to 520 bar, and a burst pressure of 860 to 1300 bar. They experience dynamic stresses owing to the changing of the drillpipes and to the ascent and descent of the drillpipes (approximately 15 m over the course of 1 hour). They have to exhibit great bending flexibility in order to withstand the significant variations in radius of curvature. On account of their accessibility, they may easily be replaced, so it is preferable for them to have an inexpensive structure. Lastly, they have to withstand great variations in pressure between the load and load-free phases.
In the past, a solution to these problems has been sought in the form of hoses consisting of short lengths of rigid or slightly flexible pipe connected by articulated joints, as shown in U.S. Pat. No. 1,852,632 and U.S. Pat. No. 1,963,368. Nowadays, the flexible pipes generally used for mud injection are bonded pipes, i.e. pipes having reinforcements embedded in an elastomer matrix. This is, in particular, the case of the pipe shown in document U.S. Pat. No. 4,514,103 cited above.
It became apparent that there was a need to diversify the current offering of mud hoses available and to seek an alternative to current bonded pipes.
An object of the invention is to propose a pipe of the unbonded type that comprises at least one pair of crossed armor plies arranged at substantially 550°, with no pressure vault, i.e. an unbonded flexible-pipe structure of reduced cost as compared to that of conventional structures with a vault. These “balanced” structures, known as “55” by specialists, are known for low-pressure applications and are not suited to withstand the pressures envisaged in the present application and thus to resist the ensuing problem of creep.
In fact, in “55” pipes with no pressure vault, the pressure of fluid transported tends to cause the pressure sheath to flow into the gaps that exist between the inner-ply armor wires. This is particularly critical when the local clearance between wires may be large, as is highlighted in document FR 2 664 019 A of the applicant, which advocates, in particular, interlocking of the inner armor ply in response to the problem posed.
Moreover, a further problem revealed by the applicant and that the invention aims to solve lies in the rotation of unbonded flexible pipes when they are pressurized. This tendency is particularly significant since mud hoses for rotary drilling are of small diameter.
This tendency to rotate is explained, in particular, by the radial swelling of the armor plies during pressurization. This radial movement of the armor plies is particularly great when the flexible pipe includes a set of layers (antiwear strips, reinforcing strips, anticreep strips, adhesive strips) that suffer crushing during pressurization. This is particularly the case of the solution advocated by the present invention in which the proposed structure includes a plurality of layers that, because they are crushed, tend to exacerbate rotation of the pipe about its axis.